Implantable cardiac devices are well known in the art. They may take the form of an implantable defibrillator (ICD) to treat accelerated rhythms of the heart such as fibrillation, or an implantable pacemaker to maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
The devices are generally implanted in an upper portion of the left-side of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode-carrying leads which are implanted within the heart. The electrodes are positioned within the heart, for making electrical contact with their designated heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to deliver the desired therapy.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
For defibrillation, one lead may include at least one defibrillation electrode arranged to be positioned in the right ventricle. When fibrillation is detected, a pulse generator delivers a defibrillating shock from the defibrillation electrode in the right ventricle to the device conductive housing to terminate the arrhythmia. Alternatively, a further defibrillation electrode may be positioned in the right atrium or superior vena cava and electrically connected to the right ventricular defibrillation electrode. In this arrangement, the defibrillating shock is delivered from the parallel connected defibrillation electrodes to the conductive housing.
Sleep apnea is a serious, potentially life-threatening condition characterized by brief interruptions of breathing during sleep. In a given night, the number of involuntary pauses in breathing (apneic events) may be as high as twenty to sixty or more per hour. During sleep apnea, blood oxygen saturation levels are reduced. These reduced blood oxygen saturation levels can be especially serious for patients with congestive heart failure (CHF).
As is known, CHF disease state may be evaluated through impedance measurements utilizing electrodes implanted in the heart. Such measurements may be carried out by applying a current between a pair of the electrodes and measuring the voltage between those electrodes or another pair of electrodes. Hence, an implanted cardiac stimulation device is well suited for such an application. Sleep apnea may also be monitored in this manner.
Implantable cardiac devices are also well suited for providing sleep apnea therapy. One such therapy is phrenic nerve stimulation (PNS). Here, stimulation pulses from the device's pulse generator are applied to phrenic nerves associated with the diaphragm or to diaphragm muscle itself. Both of these forms of stimulation therapy are included herein as PNS.
Another form of therapy which an implantable cardiac device is well suited to provide is overdrive pacing. Here, stimulation pulses are provided to the heart to increase the cardiac rate and cardiac output. The stimulation pulses may be in accordance with a pacing modality referred to as DAO pacing where both the atrial and ventricles are paced. The atrial pacing rate is above a base rate and a ventricular pacing pulse is provided an escape interval after each atrial pacing pulse. DAO pacing is considered effective at preventing central sleep apnea because the higher cardiac rate will increase cardiac output which in turn will decrease the delay in the respiratory control loop.
Sleep apnea may be defined as the lack of respiratory function for a period of time such as, for example, ten seconds. Unfortunately, not long after that lack of respiratory function, blood saturation levels may already be dangerously reduced. Hence, it is most advantageous to confirm apnea and provide therapy as soon as possible after the apnea episode may be confirmed. Otherwise, harm to the patient may result. Hence, it would be most advantageous to be able to detect sleep apnea early to enable an early sleep apnea confirmation and therapy.
Such early sleep apnea detection may be carried out as described, for example, in U.S. Pat. No. 7,371,220, filed on Jun. 30, 2004, entitled SYSTEM AND METHOD FOR REAL-TIME APNEA/HYPOPNEA DETECTION USING AN IMPLANTABLE MEDICAL SYSTEM, the content of which is hereby incorporated herein by reference in its entirety. This allows the sleep apnea to be detected before the condition has persisted too long to cause harm without intervention. Therapy may then be applied early enough to preclude serious de-saturation.
Impedance monitoring to measure respiration is generally carried out with a variable gain amplifier. In the detection of apnea, as described in the aforementioned referenced application, the impedance signal is compared to a running average or threshold. The difference between the two is an error that is accumulated. When the accumulated error equals a detection threshold, detection of apnea is declared. How soon this occurs before the apnea is actually confirmed is largely dependent upon the gain of the variable gain amplifier. If the gain is too high, the error accumulates too fast resulting in too short a time until confirmation and possible confirmation error. If the gain is too low, the error can accumulate too slowly to unduly delay apnea detection until well after apnea onset and may even preclude apnea detection all together. Hence, the present invention addresses these issues concerning apnea detection control to assure that the time from apnea detection to apnea confirmation is neither too long nor too short.